Three dimensional (3D) x-ray and transmission electron microscope imaging with computed tomography (CT) require high three dimensional positional precision when rotating the sample under investigation between the series of transmission images, also known as projections, that are generated to perform the subsequent tomographic reconstruction. The effect of unwanted sample movement due to the imperfections of the rotation axis movement between different projections must be corrected typically below the imaging resolution of the microscope to enable a tomographic reconstruction without artifacts due to misregistered projections. At least one such x-ray tool, as described in U.S. Pat. No. 7,215,736, requires that the correction of the displacement of the sample due to rotation of a sample be accurate to within tens of nanometers in all three dimensions.
Such precision is difficult to achieve in conventional rotating stages due to random errors from bearings and spindle wobble and play, as well as manufacturing variations in the motor housing and the dimension and smoothness of the stage assembly attached to the motor. Furthermore, no matter how accurate the components can be made, some portion or all of it must be constructed out of typical engineering materials, which in general have significant thermal expansion characteristics.
U.S. Pat. No. 7,535,193 describes a rotating stage assembly that can be used to perform high precision position error correction for x-ray and transmission electron microscope tomographic imaging. It continuously senses and then corrects stage rotation and translation errors. It performs these corrections by sensing using five sensors, placed to measure the adjustments for five corresponding actuators, which adjust the entire stage to a reference frame, maintaining the position accuracy of the rotation axis of the stage. This implementation represents an active spindle correction in closed loop feedback that acts as a near perfect spindle.
Another solution implemented in the past places an X/Y/Z stage, which holds the sample holder, on top of a rotation (theta) stage with a metrology reference disc. This configuration allows the sample to be positioned on the axis of rotation of the theta stage. The problem with this configuration is that the theta stage is far from sample stage, leading to higher sample location errors. Moreover, active correction using the X/Y/Z stage on top of the theta stage is generally not possible, because existing X/Y/Z stages have excessive tip/tilt and runout errors during motion beyond the positional accuracy required for an active correction of spindle errors.
Another solution places the rotation (theta) stage with the metrology reference on top of a rotation X/Y/Z stage. Here, the theta stage is closer to sample, reducing the effects of tip/tilt on sample location. The problems with this configuration are that it is not possible to center the sample on the axis of rotation and active correction not possible because stage location is measured relative to the rotation stage only.
Another solution places the rotation (theta) stage with the metrology reference disc on top of a rotation X/Y/Z φ,γ stage. A further X/Y stage is placed on top of the theta stage and this X/Y piezo stage supports the sample holder. In this configuration, active correction is possible with X/Y piezo stage in between theta rotation stage and sample. This system is costly to design and implement, however.